Apr-2025

Enhancing profitability and productivity of the molecular sieve dehydration process

Molecular sieve beds are utilised in the natural gas processing industry to remove water from natural gas streams.

Kevin Chase
Johnson Screens

Article Summary

Today’s changing energy landscape is driving a global increase in demand for natural gas supplies that is poised to continue for the long term. As producers strive to meet the current and future demand, the focus on the efficiency, capacity, and productivity of gas processing plants is more important than ever.

Many molecular sieve beds in process plants are under-designed, with obsolete internal designs that can shorten the expected life of the bed media and reduce the productive cycle time between turnaround and maintenance. Moreover, any restrictions to the flow below the bed’s support grids will also threaten plant processing capacity, efficiency, and overall profitability.

The following discussion will further delve into the transformative potential of enhancing gas dehydration vessels and the solutions that can significantly boost their efficiency, paving the way for a more productive and sustainable future in the natural gas processing industry.

Molecular sieve function
Molecular sieve dehydration is a regenerative process involving cycles between adsorption and regeneration phases regulated by switching valves. In the adsorption phase, wet hydrocarbon gas enters the top of the adsorption tower and flows downwards through the molecular sieve material, where the water is adsorbed. The essentially dry natural gas exits at the bottom and is ready for further processing or sale.

The water present in the inlet gas stream is adsorbed via contact with solid desiccant molecular sieves to very dry concentrations in gas streams. Then, hot regeneration gas flows through the molecular sieve bed to vaporise the adsorbed water and remove contaminants. After the regeneration, the bed is cooled with cool regeneration gas to the operating temperatures used in adsorption cycles.

Fixed vs variable cycling design
The primary cause of molecular sieve degradation is heating stresses; hence, the sieve’s performance decreases as the number of cycles increases. Typically, molecular sieve beds are designed with fixed cycle times. That means excess capacity is available at the beds’ start of life since the bed heights are designed for end-of-life conditions.
Therefore, the bed’s lifetime can be extended by reducing the total number of cycles via the implementation of variable cycling design (VCD). In VCD, the cycle time is adjusted at regular intervals based on adsorption capacity, which is determined by breakthrough testing.
Life of the molecular sieve bed

The amount of molecular sieve material in the vessels also impacts the life of the bed. Adding more adsorbent material makes it possible to extend cycles and the life of the bed. However, the traditional flat surface design grid assemblies impose some limits to the volume of the molecular sieve bed in two ways.

These support screens are traditionally located near the bottom tangent line of the vessel head. The grid leaves the entire head volume as a dead area for material placement, with no reaction or drying adsorption occurring in this space. In addition, the complete load (molecular sieve load and pressure drop) is supported by the grid and transferred to the vessel’s shell through beams and a support ring. That limits the molecular sieve load and creates a failure point for the assembly.

Also, in traditional support grids, dust/debris may get trapped in the support grid itself or between the vessel shell and the edge of the grid. With no way for debris to ‘escape’, it tends to accumulate in the gap, reducing the grid’s allowance for thermal expansion. This can potentially lead to grid failure, media leakage, and the need for unplanned maintenance.

Rethinking support grid design
Recognising the pivotal role of the support grid design in the molecular sieve bed life and process economics, a reimagined design not only improves liquid and gas flow, bed utilisation, and distribution but also maximises reliability, thereby reducing maintenance needs and instilling confidence in its performance.

The new concept design places the grid directly on the bottom head surface of the vessel, filling the entire volume with media. By doing that, the vessel head supports the grid directly and creates a strong and rigid structure without adding a special ledge ring or heavy beams to the vessel.

This design not only increases the bed volume, allowing for higher process capacity, but also improves liquid and gas flow, bed utilisation, and distribution. These improvements maximise reliability, reduce maintenance needs, and ultimately enhance the overall efficiency and profitability of the gas processing plant.

Although the same increase in bed volume could be achieved by using conventional outer baskets, the flow patterns through the bed would result in inefficient media utilisation. By splitting the grid into totally enclosed elements with a bolted and gasketed connection to the central hub, we ensure that each one collects the flow on a larger area than ‘standard’ outlet baskets covered with inert balls. Additionally, this expanded collection area does not create flow turbulence like conventional support grids.

Another advantage of the split element design of the proprietary Shaped Support Grid (SSG) is that the thermal expansion is greatly reduced compared to a traditional support grid. The thermal expansion of the assembly is only as great as the width and length of each panel. Such contained thermal expansion inside the ceramic ball bed avoids the creation of fines or ceramic breakage, ensuring the longevity and reliability of the molecular sieve bed. Also, the assembly expands and contracts under the bed without compromising an outer perimeter seal, which may happen in a cyclic gas dehydration application. The enclosed stainless steel bottom surface panel ensures any bed material that might migrate under one of the panels will not leak into the flow of the process.

Solution in action
The Johnson Screens Shaped Support Grid design (see Figure 1) has been successfully deployed in numerous natural gas processing plants worldwide, either in new installations or in retrofits and expansions of existing ones. Its proven effectiveness in real-world applications serves as a testament to its reliability and the inspiration it provides for future advancements in the industry.

An example of a successful deployment is in a liquefied natural gas (LNG) production plant in South America in 2017. One of its trains incurred significant production losses due to high differential pressure across the molecular sieve beds. This issue arose because of a restriction below the beds’ structural supports coupled with the fact that the supports were grossly under-designed for load bearing.

Additionally, the operator pursued reducing the frequency of planned turnarounds to once every four years, which was challenged by their bed design life of three years and potential pressure drop (dP) issues after two years. The SSGs replaced the then-current support grid during a planned turnaround. As per the design, they was installed directly onto the bottom head of the vessels, compared to the previous support grates, which were installed where the vessel’s cylindrical section meets the vessel head.
The SSG installation increased the design dP of the bed supports by more than three times the original limit, reducing the risk of production loss and bed support failure. To assure that the SSG’s technical design was acceptable and within the constraints of the existing facility, third-party computational fluid dynamics (CFD) analyses and finite element analyses (FEA) were performed to determine the flow patterns through the beds, SSG, and outlet nozzle and determine new loading and stress resultants on the lower vessel head and nozzle, respectively.

The increased bed depth available with the placement of the SSG allowed approximately 25% more molecular sieves to be loaded into the space created in the vessel head when the SSG was installed. The original regeneration parameters, i.e., regeneration gas flow/temperature, were adequate to effectively regenerate the beds within the same time period.

After installing the SSG and increasing the amount of material in the beds, the start-up dP across the ‘fresh’ beds was approximately 45% greater. With the SSG enhancements, the operator then implemented a variable cycling regime in the train. Based on breakthrough testing, the adsorption time was increased from 18 to 29 hours.
By adopting the new SSG design, the operator met the objectives of reducing the risk of production loss. The plant can now operate at full capacity without being restricted by high bed dP. The installation also reduced the molecular sieve bed change-out frequency from three to four years while reducing the risk of support grid failure leading to unplanned downtime.

In addition, reducing the annual firing hours of the regeneration gas heaters increased the adsorption time by reducing the number of regeneration cycles, resulting in an approximately 13% reduction in greenhouse gas emissions. Another positive impact was reducing the quantity of molecular sieve material that must be disposed of, thanks to less frequent bed changes in the long term.

Conclusion
Upgrading to the SSG and implementing VCD have proven to be effective solutions for improving a plant’s processing capacity, production efficiency, and profitability.

This short case study originally appeared in PTQ's Technology In Action Feature - Q2 2025 Issue

For more information: fletcher@johnsonscreens.com