Gas Dehydration-Molecular Sieve
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Molecular Sieve (Beads) Adsorbents
Molecular sieves are crystalline materials with highly porous structures that possess the unique ability to selectively adsorb molecules based on their size and polarity. These sieves are typically composed of aluminosilicate minerals, zeolites, or synthetic materials. Their porous nature provides a large surface area, allowing them to trap water molecules while permitting the passage of natural gas molecules. This property makes molecular sieves an ideal choice for efficient dehydration processes.
Why utilize Zeochems' Molecular Sieve Adsorbents?
Through the existing technical partnership with Zeochem, Mountleigh provide the leading edge of synthetic molecular sieve technology. We offer an ultra-stable molecular sieve able to withstand thousands of thermal regenerations with delayed decay and exceptional capacity retention. As the oil and gas industry continues to evolve, we believe Zeochems’ molecular sieves will play a crucial role in ensuring the quality, safety, and sustainability of natural gas transportation and utilization.
We offer the following support:
- What should be done when a sieve unit upset occurs?
- How do I know when it is time to reload and is a breakthrough test required?
- What are the most important things to do when preparing to reload the molecular sieve beds?
- When should I consider switching from 4A to 3A molecular sieve?
- What causes pressure drop across the sieve bed to increase?
- How do you optimize the life of the molecular sieve?
Our technical sales and service teams are available to answer questions and find solutions for your adsorption needs via our website, www.mountleigh.com.ng. We strive to be your partner and pride ourselves on our responsiveness.
The 3A molecular sieve has a pore opening of approximately 3 angstroms. Type 3A is made by ion-exchanging the sodium in the type 4A sieve with potassium. The 3A molecular sieve will exclude most molecules except water, making it very selective. This type of bead also has advantages in crush strength, durability, and high rate of adsorption.
The 4A molecular sieve is an alkali metal aluminosilicate with an effective pore opening of approximately 4 angstroms. The sodium form of type A is widely used as a general-purpose drying agent and has good physical and adsorption properties. Type 4A beads can be used to adsorb water, ammonia, methanol, ethanol, and carbon dioxide.
The 5A molecular sieve is an alkali metal aluminosilicate with an effective pore opening of 5 angstroms and is the calcium-exchanged form of the type A zeolite. The strong ionic forces of the divalent calcium cation (Ca2+) make 5A an excellent choice for removing carbon dioxide, carbon monoxide, alcohols and other hydrogen sulphide, and other polar molecules. This type of molecular sieve has a high rate of adsorption and desorption, a higher rate of contamination resistance and a high crush strength.
FAQs About Molecular Sieve
Typically, there are some options that allow continued operation in the plant until a change out can occur. Adjustments to the cycle times, regeneration and feed conditions, and flowrates are sometimes available and can be made. The last thing considered is a reduction in the feed flow rate once all else has been done and further adjustment may be necessary. Zeochem can help with recommendations and a prioritized plan of action.
For short term shutdowns of less than 1 day, especially in warm climates, the vessels can be locked in under full pressure and simply restarted from the point of stoppage. In cold climates or wintertime conditions, 6-12 hours could be a downtime limitation due to more rapid cooling. In such cases, depressurizing or partial depressurizing of the vessels and surrounding piping and equipment would be recommended in order to avoid condensation and liquids during the shutdown. For upstream pipelines and equipment that cannot be depressurized, low point drains should be checked to make sure any collected liquids are drained off before restarting. For long term shutdowns of more than 1 day, it is recommended to regenerate all the beds first, depressurize down to 5-10 psig while maintaining a blanket of dry inert gas in each vessel. Check the vessel pressures periodically in order to maintain positive pressure to avoid air ingress. Note that any beds in a regeneration heating cycle should be completed prior to shutdown in order to avoid repeating the regeneration heating step from the beginning.
When possible, first regenerate the sieve beds to ensure dryness. Normally a short 70-80% heating cycle is sufficient for this purpose given the sieve will not be as wet as during normal operation. If initial regeneration is not possible or if the sieve is loaded under sufficiently dry conditions, the unit can be started up on regular feed at 50% flow rate while simultaneously starting a regeneration cycle of 1 of the beds. As soon as the regeneration is completed, switch beds and start regeneration of another bed. Once all the beds have been regenerated, ramp up the feed rate to full design rate and adjust the cycles times to the technical recommendations.
When beds are adsorbing in parallel and there is more flow restriction in one vessel than the others, the flow will automatically balance between the beds to achieve an equivalent pressure drop. If it is minor, then it is ordinarily not an issue. If the flow imbalance is large enough, early breakthrough on the vessel with the high flow rate (lowest pressure drop restriction) can occur. In order to better balance the flow rate between the vessels, feeds must be adjusted. This is normally done by adjusting (partially closing) valves, either manual shutoff valves or the valve travel of automatic valves. It can be a trial and error process to achieve balance so it should occur as a series of small adjustments until the operation noticeably improves. In cases where this is not possible, adjustment of the cycles and/or conditions may be possible to prevent breakthrough. When all else fails, the inlet feed rate must be reduced until breakthrough no longer occurs.
There are several advantages with use of 3A sieve that can potentially benefit the plant operation. 3A sieve is essentially a drop-in replacement that gives the same or better performance than 4A sieve in many applications. Only very small compounds such as ammonia and water easily pass through the 3A crystal pore openings for adsorption. If adsorption of other contaminants such as sulphur, CO2, and oxygenates is a concern, the 3A sieve minimizes co-adsorption of these compounds and helps maintain constant and stable levels of these contaminants in the product and regeneration streams, avoiding spikes and problematic levels. 3A sieve also regenerates more thoroughly at lower temperatures with a typical target of 450°F (232°C). This saves energy and minimizes carbon laydown/coking. During each regeneration heating cycle, a small amount of carbon/coke is deposited on the sieve and over time, it causes adsorption capacity decline. Minimizing this thus helps to achieve longer sieve life.
Schedule the change out well ahead of time, preferably during an already scheduled shutdown or turn around. Order and have all needed products and supplies on site well ahead of time to avoid any delays. Make sure all contractors, plant personnel, and equipment will be ready to begin the morning of the scheduled start, with any necessary orientation, training, etc. completed in advance. Follow the Zeochem guidelines and recommendations for unloading and reloading the sieve to streamline the process and avoid delays. Have contingency plans in place should there be weather or unexpected delays that occur.