PTQ Q2 2025 Issue

Technology in Action

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 poten- tially 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 rei- magined design not only improves liquid and gas flow, bed utilisation, and distribution but also maximises reliability, thereby reducing maintenance needs and instilling confi - dence 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 improve - ments 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 ele- ments 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 propri- etary Shaped Support Grid (SSG) is that the thermal expan- sion is greatly reduced compared to a traditional support grid.

Enhancing profitability and productivity of the molecular sieve dehydration process

Molecular sieve beds are utilised in the natural gas pro- cessing industry to remove water from natural gas streams. 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 pro- ductive cycle time between turnaround and maintenance. Moreover, any restrictions to the flow below the bed’s sup - port grids will also threaten plant processing capacity, effi - ciency, and overall profitability. The following discussion will further delve into the trans- formative potential of enhancing gas dehydration vessels and the solutions that can significantly boost their effi - ciency, paving the way for a more productive and sustain- able 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 adsorp- tion 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 regen- eration, 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 heat- ing 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 reduc- ing 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.

PTQ Q2 2025