Practical considerations for the design of adsorbent beds

Refineries have to achieve outstanding reliability and in order to obtain this, they must ensure they optimise the lifetime of molecular sieves. Oil and gas facilities have to achieve outstanding reliability in their various processes in order to be cost effective and honour their commitments.

Alexandre Terrigeol, CECA Molecular Sieves
Olivier Trifilieff, Pall Corporation

Viewed : 2002

Article Summary

An unplanned shutdown is a loss of millions of dollars. Adsorbent beds are commonly used in refineries, petrochemical and gas plants, to dry and purify various streams. They face the same logic and in practice have to perform during a set lifetime without a hitch and sometimes, even while bearing slightly different operating conditions than design. Taking the example of natural gas drying with molecular sieves, the dehydration unit is typically designed by the molecular sieve manufacturer.

This does not only involve choosing the right adsorbent(s), but also designing the adsorption and regeneration procedures so that the beds will be immediately efficient and resistant at once. Upstream of the dehydration unit is raw gas that goes through a series of purifications and treatments. Before entering the molecular sieve towers, it has to be free of liquids.

To do this, specialised filtration and separation companies design and implement dedicated equipment that mechanically separate liquid droplets from the gaseous stream. Prepared jointly by CECA SA and Pall Corporation, two leading companies in their respective fields, this paper describes the main contaminants, threats and constraints that are likely to damage a dehydration unit in case of a poor design leading to the presence of liquids. It explains how experienced suppliers anticipate the risks as they design the unit and its surroundings to ensure optimal performance that goes beyond the EPC and owner expectations.

Oil and gas facilities have to achieve outstanding reliability in their various processes in order to be cost effective and honour their commitments. In that regard, the vast majority of plants around the world operate in a satisfactory manner, although there is always room to improve processes, and eventually to minimise OpEx and to maximise profitability. Adsorbent beds are commonly used in refineries, petrochemical and gas plants to dry and to purify various streams. In the more critical processes that include a cryogenic section, such as NGL recovery plants and LNG production plants, the gas is dehydrated down to typically less than 1 ppmV water: a very low specification that only molecular sieves can achieve. The efficiency and the lifetime of a molecular sieve dehydration unit can be enhanced by paying more attention to the design of the unit itself and of its surroundings. Field experience greatly helps analyse a process specification to anticipate the risks. This paper focuses on the potential operation issues related to the presence of liquids in the adsorbent beds, whether these liquids are carried over from the upstream liquid/gas separator, or are due to inappropriate regeneration procedures. It does not explicitly deal with chemical contaminants in the gas phase (oxygen, acids, etc.) which were described in a previous paper by CECA.1 A two-step approach is implemented.

First the liquid/gas separation unit upstream of the driers has to be designed and specified correctly to minimise liquid carry-over. Second, the molecular sieve dehydration unit itself has to incorporate the right type of adsorbent(s) as well as the regeneration procedures to make beds efficient and resistant at once and in the long run.

Basics of Industrial Adsorption
Molecular sieves are alumino-silicate adsorbents made from a porous material in powder form called a zeolite. They can be shaped into beads and pellets of a few millimetres due to binding clays. This material shows a crystal structure with channels and pores, and a wide electronically active surface area. Because of these characteristics, zeolites have the ability to selectively capture and retain (‘adsorb’) polar molecules that are small enough to enter their pores. The lower the temperature, the higher the adsorption capacity. Reciprocally, when the temperature is increased, the molecules are desorbed. It is therefore possible to use zeolites to selectively sift impurities from a fluid until they are saturated and then to desorb these impurities to recover fresh adsorbent. These two steps are referred to as ‘Adsorption’ and ‘Regeneration’. Different zeolites can be used depending on the application. This paper will focus on natural gas drying for which, typically, ‘4A’ type zeolites are used (zeolites that show a pore opening of approximatively 4 angstroms).

Molecular sieves are loaded in vertical adsorbers. The gas to be dried goes through the adsorbers, and water is captured along the way. Within a few hours (12 to 36 hours depending on the size of the adsorption unit), a given bed is saturated with water and it has to be regenerated. Therefore, at least two beds have to be used: one in the adsorption phase, while the other is being regenerated. In order to accommodate high flow-rates and acceptable diameters, several adsorbers are often used in parallel with a time lag during which they are alternatively regenerated (Figure 1).

Molecular sieves are efficient and robust materials. Aside from proportional sizing of vessels, and ensuring hydrodynamic constraints are accounted for, there are however two key pitfalls to be avoided in order to perform in an optimised manner. Both relate to the presence of liquids.
• The incoming gas must be free of liquids which can strongly react with the structure of the molecular sieve or foul the porosity.
• The formation of liquids in the regeneration process should be avoided. The most suitable and gentle regeneration procedure has to be implemented. Moderation of the process yields the best result.

Figure 2 is a schematic view of a typical line up showing the two units discussed in the next chapters:
• The liquid/gas separation unit that prevents liquids to be carried over onto the molecular sieves
• The adsorption unit itself which has to be efficiently and gently regenerated

The inlet gas entering the liquid/gas separation unit may have several origins (raw gas, acid gas removal unit, physical solvent, heat exchanger, caustic soda washing, etc.) and therefore, contains liquid droplets of different nature (water, hydrocarbons, caustic soda, amine, etc.). With regards to regeneration, it can be achieved by the dry gas itself, or by another gas, preferentially clean and dry (fuel gas, boil off gas, etc.).

Preventing liquid ingression and managing gentle regeneration are two important issues that are well known to plant operators and EPC companies in charge of new projects. However, to avoid any trouble in real operation, they must be addressed very carefully during the design phase. Best Practices can be discussed and implemented with the help of experienced filtration technology suppliers and adsorbent manufacturers.

Typical Operation Issues
For a given lifetime, a good design of the dehydration unit guarantees a specified dew point during a given adsorption time and a maximum pressure drop. However, these parameters are affected by the presence of liquids. The main consequences of poor operation are premature water breakthrough and fast pressure drop increase. Ultimately these may significantly affect the lifetime of the dryer and would require a change out of the product, earlier than designed.

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